Gaining

To answer the question, I would (personally) recommend enjoying and loving your body the size that it is. If you want, you can engage in "passive" gaining -- which means basically keep eating and indulging. This can result in a slower, steadier, and usually smaller weight gain over time.

You also have to take the doctor's word with a grain of salt. I've known people who are in their 60's and 70's now whose doctor told them they would be dead by 30. If you have a specific situation, work to fix it. Things like HBP, etc.